1. Technical Field
This invention relates in general to electronic circuits and, more particularly, to RC filters.
2. Description of the Related Art
RC-active filters need calibration because of the poor control on the absolute values of integrated resistors and capacitors. In CMOS technologies, these spreads may produce variations in the RC products as high as ±50%. State-of-the-art processes have high-poly resistance spreads in the range 225-425Ω/□ (±30%) and the poly1-poly2 capacitance in the range 1.15 -1.65 fF/μm2 (±17%).
In general, an RC-active filter must have a fairly precise, and predictable, filtering curve. Variations in the RC products due to technology spreads have a strong impact on the shape of the filter. The possible variation in filter shape due to variations in the RC product is usually not acceptable and thus filter calibration is often mandatory.
A technique for automatically tuning integrated circuit RC-active filters is set forth in J. B. Hughes, N. C. Bird, and R. S. Sohn, Self-Tuned RC-Active Filters for VLSI, Electronic Letters, 11 Sep. 1986, Vol. 22, No. 19, paper 6—6 (hereinafter “Hughes”). This paper proposes a separate on-chip monitor circuit to evaluate and tune the RC product of an RC-active filter using adjustable capacitor arrays. However, this filter has an asymmetrical coverage range (−33% to +100%) that does not cover all the entire theoretical RC product variation range.
Therefore, a need exist for an RC-active filter with a range that covers all product variations.